1. Field of the Invention
This invention relates to an industrial robot's articulate device.
2. Prior Art
FIG. 1 is a vertical sectional view showing one example of a conventional industrial robot's articulate device. In FIG. 1, reference numeral 1 designates a first arm having an electric motor 3 and a harmonic device 4 at the articulated end thereof. The electric motor 3 is to rotate a second arm 2. The harmonic drive 4 is to lower the output speed of the motor 3 to transmit the torque of the latter 3 to a transmission shaft 5, to the end of which the second arm 2 is secured with bolts 20. The transmission shaft 5 is coupled to the output shaft of the harmonic drive 4 with bolts 17, and rotatably supported by the first arm 1 through a bearing 6. Further, in FIG. 1, reference numerals 14 and 15 designate bearing retaining plates which fixedly support the bearing 6 with bolts 18 and 19; 13, a dog for detecting a second-arm overrunning, secured to the outer cylindrical wall of the transmission shaft 5; 23, a limit switch provided on the lower surface of the first arm 1; and 24, a limit switch fixing bolt. FIG. 2 is a cross sectional view of the transmission shaft shown in FIG. 1. In FIG. 2, reference numeral 22 designates a limit switch for clockwise rotation overrunning; and 23, a limit switch for counterclockwise rotation overrunning.
The operation of the articulate device thus constructed will be described.
The rotation of the motor 3 disposed on the first arm 1 is applied to the input shaft of the harmonic drive 4, so that, with the speed of rotation of the motor 3 lowered, the torque of the motor 3 is transmitted through the output shaft of the harmonic drive 4 and the transmission shaft 5 to the second arm 2 to rotate the latter 2. As was described above, the dog 13 is secured to the transmission shaft 5. Therefore, when the second arm 2 is rotated beyond the allowable range of rotatory angles, the dog 13 is caused to strike against the limit switch 22 or 23 for detecting the overrunning operation, secured to the first arm 1, so that the limit switch 22 or 23 outputs an overrunning detection signal. The overrunning detection signal thus obtained is applied to a robot controller (not shown), so that the operation of the second arm (or the operation of the robot itself) is stopped immediately. This method eliminates a variety of difficulties which otherwise may be involved when the second arm is turned beyond the allowable range of rotatory angles. FIG. 3 shows the case where the dog 13 strikes against the limit switch 22 with the second arm 2 rotated clockwise (in the direction of the arrow).
In the case of FIGS. 4 and 5, the limit switch 22 is not used as the limit switch for overrunning detection; that is, for the purpose of increasing the allowable range of rotatory angles to 360.degree. or more, the limit switch is so designated as to count the number of the passage of the dog 13. In this case, the original position of the second arm 2 with respect the position of the limit switch 22 is determined as shown in FIG. 4, and how many times the dog 13 strikes against the limit switch 22 and the direction of rotation of the second arm in the operation are stored in memory. At the end of the operation, the second arm 2 is turned in the opposite direction to the original position. Therefore, in this case, the second arm 2 can be turned more than 360.degree. to the extent where the cables connected between the first arm 1 and the second arm 2 are twisted.
In another example of the articulate device, as shown in FIG. 6, a gear 5a is mounted on the transmission shaft 5, a detecting shaft 26 is rotatably coupled to the first arm 1 through a bearing 27, a gear 25 twice as large in radius as the gear 5a is mounted on one end portion of the detecting shaft 26 in such a manner that the gear 25 is engaged with the gear 5a, and a dog 26a is connected to the other end portion of the detecting shaft 26. Limit switches 22 and 23 are arranged in the same manner as those in FIG. 2.
In this case, the speed of rotation of the detecting shaft 26 is half of the speed of rotation of the transmission shaft 5. Therefore, when the transmission shaft 5 is turned through 360.degree. from a predetermined point, the detecting shaft 26 is turned through 180.degree.. Accordingly, if the limit switches 22 and 23 are set at the points which are different by 180.degree. in angular position (phase), then not only the position of the transmission shaft 5 but also the state (the presence or absence of a phase difference of 360.degree.) of the transmission shaft 5 can be detected by determining which of the limit switches 22 and 23 is operated. Thus, the second arm 2 can be rotated more than 360.degree..
The conventional industrial robot's articulate device is constructed as described above. Therefore, when the allowable range of rotatory angle of the second arm 2 is set to more than 360.degree. (for instance to 400.degree.), the second arm 2 can take two states different by 360.degree. in phase when posed in a certain posture. The robot system must store the two states in some form so that the rotation of the second arm 2 beyond the allowable range is prevented. However, if interruption of the power supply occurs for some cause, the memory of the two states by the electrical means is erased, and therefore even if the power is supplied again, only the position data is obtained; that is, the two states different by 360.degree. in phase cannot be discriminated. This may result in the following difficulty: The second arm 2 is further turned in the same direction beyond the allowable range of rotatory angles, so that the cables connected between the first and second arms 1 and 2 are broken being twisted.
This difficulty may be eliminated by modifying the articulate device as shown in FIG. 6. However, the method is still disadvantageous in that, since the limit switch together with the speed reducing mechanism and the detecting shaft which are additionally provided must be arranged at the articulated part, the articulate device is rather intricate in construction and large in weight.